Modern integrated circuits, such as monolithic semiconductor devices formed on substrates of Group IV materials such as silicon or germanium, or Group III-V materials such as gallium arsenide, or combinations of such materials, are fabricated using extremely complex processes. These processes can be generally categorized into a few different groups, such as photolithographic, deposition, and etching. Process steps that fall into one or more of these different groups are applied over and over again, forming the integrated circuit layer by layer, until it is completed.
Because both the integrated circuit itself and the process by which it is formed are so complex, there are innumerable ways in which defects and flaws can creep in to the fabrication process. Such defects are able to not only degrade the ability of the integrated circuit to function properly, but can reduce its anticipated lifetime, or cause it to not function at all. These defects can be related to a myriad of different sources, such as materials issues, handling issues, and process capability issues.
Because of the great number of potential pitfalls during integrated circuit fabrication, and the extreme cost associated with the defects caused by such, it is very important to become aware of defects and identify their sources as soon as possible. In this manner, there might be some type of remedy or rework that can be timely applied to the integrated circuits that exhibit the defects, or more likely, the source of those defects can be corrected as soon as possible, so that additional integrated circuits are not impacted by the problem.
Thus, in-line inspections are an important part of the integrated circuit fabrication process. These inspections are conducted at many different points during the fabrication process, and in some instances are conducted virtually after each individual process step. In this manner, defects and their sources are hopefully detected and identified in a timely manner, before too many integrated circuits are affected.
One important classification of such inspections are optical inspections, meaning inspections that are intended to identify defects that can be seen in some manner. These optical inspections have traditionally been done manually, meaning that a human inspector looks at the substrate, typically called a wafer, on which the integrated circuits are formed. First, an inspection may be conducted with the naked eye, which hopefully detects large defects, or large patterns of defects. Next, the inspector may look at the substrate under some type of microscope to determine additional information about the nature of the defects, or to detect defects which cannot be observed by the naked eye.
Unfortunately, such manual inspection of substrates is somewhat insufficient. For example, such manual inspection is extremely tedious to perform. Thus, human inspectors tend to tire and stop noticing the more subtle defects. In addition, due to the difference in the training, experience, and ability from one inspector to the next, the data that is produced in this manner tends to be extremely difficult to integrate into a production system that can use the data to identify problems and improve processes.
For this reason, various automated optical inspection methods and analysis systems have been developed. Unfortunately, such systems tend to be very limited in their capabilities as compared to a human inspector, generally because of their more limited cognitive and associative abilities as compared to a human. Thus, such automated optical inspection and analysis systems often miss things that an experienced and careful human inspector would find.
Thus, well trained and alert human inspectors tend to recognize and identify defects better, but automated systems are less subjective and more repeatable. What is needed, therefore, are automated analysis methods that increase the ability of an automated inspection and analysis system to recognize the sources of defects.